I posted the video because I know a lot of guys and gals that have their engine balanced at a shop would like to see how its done. The round slugs in the counter weight ? they are Heavy metal. The balancer tells you where to put the weight or take it out and how much.
He marks the crank and press fits the weight or mills metal out. The computer tells you what size hole and how deep depending on the info you input. All the slugs are tig welded in place when it done.